This invention relates to techniques for the manufacture of large optics, for example, as used in lasers for inertial confinement fusion power plants, and in particular to the manufacture of mirrors for lasers for use in such fusion power plants. The invention, however, generally has applicability to the planarization of optical substrates to overcome defects in such substrates used for laser mirror coatings and defects caused during the coating deposition process.
The National Ignition Facility (NIF) is a laser-based inertial confinement fusion research machine located at the Lawrence Livermore National Laboratory (LLNL) in Livermore, Calif. NIF uses lasers to heat and compress a capsule of deuterium and tritium (DT) fuel to the temperatures and pressures to cause a nuclear fusion reaction. In NIF a bank of 192 lasers fires a hohlraum holding the capsule. The lasers used in NIF are large, extremely powerful lasers, producing beams on the order of a foot square.
Inertial confinement fusion power plants using the technology now being developed at NIF have been proposed. The equipment, systems and support necessary for the deployment of such a fusion power plant are now being investigated and designed at LLNL. In the indirect drive approach to inertial confinement fusion (often “ICF” herein) proposed for such power plants, hohlraums, each with a capsule containing the DT fuel, are injected into a fusion chamber. As they arrive at the center of the chamber, the “targets” are fired upon by a bank of lasers. The hohlraum absorbs and re-radiates the energy of the laser beams striking the inside of the hohlraum as x-rays onto the fuel capsule. This causes the outer surface of the fuel capsule to ablate, compressing and heating the DT fuel to cause a fusion reaction.
The lasers used in such a system operate at high energies with concomitant heat and energy demands imposed on the components of the laser. As such they impose unique design requirements on the optical components within the laser. Of particular concern here is that multilayer optical coatings are laser fluence limited by small inclusions on the substrate or imbedded within the coatings. These inclusions are created by micron-sized particulates on the optical component substrate. The particulates result from imperfect substrate cleaning, contamination during transport of the substrates after cleaning to the coating apparatus, as well as other causes. The geometry of these inclusions and the interference nature of multilayer coatings can lead to extremely high light intensification around the defect, thus causing the defect to have a much lower laser resistance than the surrounding non-defective multilayer coating.
Various approaches have been tried to minimize defects in smaller optical components. In particular, for extreme ultraviolet lithography, small nanometer-sized contaminants are detrimental to the functionality of the resulting masks. A variety of approaches have been tried, some with success in addressing this issue. See, e.g., “A Silicon-Based, Sequential Coat-and-Etch Process to Fabricate Nearly Perfect Substrate Surfaces,” Mirkarimi et al., Journal of Nanoscience and Nanotechnology, July, 2005, and “Advancing the ion beam thin-film planarization process for the smoothing of substrate particles,” Mirkarimi et al., Microelectronic Engineering 77 (2005) 369-381. Each of these publications describes techniques for mitigating surface imperfections such as pits or particles in the coatings for extreme ultraviolet lithography masks. The approaches described in the articles, however, address defects which are much smaller than those of concern here. For example, the techniques described in these articles address defects on the order of a few tens of nanometers in depth, as opposed to the micron-sized defects problematic with the optical and near infrared coatings. Furthermore, the materials used in the processes described in these articles, primarily silicon, are highly absorbent of energies at the wavelengths of the laser light. As such they cannot be employed for components in which wavelengths in this range are used. Finally, the primary concern addressed in these prior art approaches is one of assuring reflectivity and surface flatness for mask transfer. Here, in contrast, the primary issue relates to energy concentrations around the defects within the coating.
What is needed is a technique for mitigating nodular defects of approximately micron size in a manner to enable use of optical components that include such defects in laser or optical applications.